Real image mode variable magnification finder

ABSTRACT

A real image mode variable magnification finder has an objective optical system with positive refracting power, an image erecting optical system, and an ocular optical system with positive refracting power. The objective optical system has a first lens unit with negative refracting power, a second lens unit with negative refracting power, a third lens unit with positive refracting power, and a fourth lens unit with negative refracting power. When the magnification of the finder is changed, a plurality of lens units including the third lens unit are moved. In this way, the total length of the objective optical system can be reduced, and thus a compact finder is obtained.

This is a Divisional of application Ser. No. 09/087,964, filed Jun. 1,1998 now U.S. Pat. No. 6,256,144, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a real image mode variable magnificationfinder for attachment which is constructed to be independent of aphotographing optical system as in a still camera or a video camera.

2. Description of Related Art

Real image mode variable magnification finders have been designed sothat an intermediate image is formed inside an image erecting system toreduce the total length of an objective unit. As an example, a design isknown that a Porro prism is divided into two pieces. When this design isused, as shown in FIG. 1A, the entrance window of an objective opticalsystem 2, in contrast with FIG. 1B, can be located at a position lowerthan the window of an eyepiece 4. Thus, a shift S_(h) of the objectiveoptical system 2 from the optical axis can be diminished to keepparallax with a photographic lens 7 to a minimum. In particular, it isknown that a prism such that light from the objective optical system 2is reflected upward and then back minimizes interference with a filmmask 5 of a camera and is most suitable for use in reducing thethickness of the camera. Also, in FIGS. 1A and 1B, reference numeral 1represents a finder unit; 3, a Porro prism; 6, a film; and 8, a cameracase.

As will be obvious from Japanese Patent Preliminary Publication Nos. Hei7-84184 and Hei 9-68739, it is known that, in order to place a mechanismmember for changing the size of a field frame, the field frame is placedabove the side face of an objective optical system including a prism,after light is reflected three times by the prism, and thereby space forthe mechanism member can be provided.

Further, as set forth in Japanese Patent Preliminary Publication No. Hei5-53054, it is known that, in order to increase the optical path lengthof the back focus section of the objective optical system, it is onlynecessary to use a prism whose entrance surface is concave.

In addition, as disclosed in Japanese Patent Preliminary Publication No.Hei 1-257817, a technique is known that, in a real image mode finderusing a Porro mirror in an image erecting optical system, an eyepiece isfixed to a frame to prevent dirt particles from penetrating into anintermediate image, providing an enclosed structure.

However, each of Hei 7-84184, Hei 5-53054, and Hei 9-68739 which arementioned above has the problem that the adhesion of dirt particles to afield lens located on the pupil side of the intermediate image cannot beprevented because the eyepiece is movable for diopter adjustment.

Japanese Patent Preliminary Publication No. Hei 4-51108 is capable ofincreasing the back focal distance of the objective optical system, buthas the problem that the total length of the objective optical systembecomes large and thus the thickness of the camera cannot be decreased.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a realimage mode variable magnification finder which has an objective opticalsystem whose back focal distance is long and whose total length isshort, rarely allows the penetration of dirt particles althoughdiopter-adjustable, undergoes little change in performance, and is smallin size.

In order to accomplish this object, according to the present invention,the real image mode variable magnification finder includes an objectiveoptical system with positive refracting power, an image erecting opticalsystem, and an ocular optical system with positive refracting power. Theobjective optical system has a first lens unit with negative refractingpower, a second lens unit with negative refracting power, a third lensunit with positive refracting power, and a fourth lens unit withnegative refracting power, and is designed so that when themagnification of the finder is changed, at least one lens unit, namelythe third lens unit is moved.

Further, according to the present invention, the real image modevariable magnification finder is constructed so that the image erectingoptical system includes a prism and the fourth lens unit with negativerefracting power is configured to be integral with the entrance surfaceof the prism.

Still further, according to the present invention, the real image modevariable magnification finder includes an objective optical system withpositive refracting power, an image erecting optical sytem, and anocular optical system with positive refracting power. The image erectingoptical system is composed of a prism and a mirror, and the ocularoptical system is provided with at least two lenses, that is, a fixedlens and a moving lens.

This and other objects as well as the features and advantages of thepresent invention will become apparent from the following detaileddescription of the preferred embodiments when taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing cases where different, conventionalreal image mode variable magnification finders are incorporated incameras;

FIG. 2 is a view for explaining the reflection of light caused by aprism used as a reflecting member on the ocular optical system side;

FIGS. 3A, 3B, and 3C are sectional views showing arrangements, eachdeveloped along the optical axis, at wide-angle, middle, and telephotopositions, respectively, of the real image mode variable magnificationfinder of a first embodiment in the present invention;

FIG. 4 is a perspective view showing the configuration of a Porro prismused in the real image mode variable magnification finder of the presentinvention;

FIGS. 5A, 5B, and 5C are diagrams showing aberration characteristics atthe wide-angle position of the finder in the first embodiment;

FIGS. 6A, 6B, and 6C are diagrams showing aberration characteristics atthe middle position of the finder in the first embodiment;

FIGS. 7A, 7B, and 7C are diagrams showing aberration characteristics atthe telephoto position of the finder in the first embodiment;

FIGS. 8A, 8B, and 8C are sectional views showing arrangements, eachdeveloped along the optical axis, at wide-angle, middle, and telephotopositions, respectively, of the real image mode variable magnificationfinder of a second embodiment in the present invention; and

FIGS. 9A, 9B, and 9C are sectional views showing arrangements, eachdeveloped along the optical axis, at wide-angle, middle, and telephotopositions, respectively, of the real image mode variable magnificationfinder of a third embodiment in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before undertaking the explanation of the embodiments, a descriptionwill be given of the general function of the real image mode variablemagnification finder according to the present invention.

For the objective optical system of the real image mode variablemagnification finder, it is necessary to improve a retrofocus propertyand strengthen a forward negative power in order to obtain a back focaldistance as long as possible. In the present invention, since theforward negative power is shared between the first and second lensunits, one lens unit need not have a higher power than is necessary, andthus curvature of field and distortion can be minimized. Divergent lightemerging from the second lens unit is collected by the third lens unitwith positive power. The fourth lens unit with negative power, locatedbehind it, has two effects. One of these is that the back focal distancecan be increased by the diverging action of the fourth lens unit. Theother is that the pupil position of the objective optical system isshifted forward, and the diameter of a front lens can be diminished.

The magnification of the finder is changed by chiefly moving the thirdlens unit. Specifically, the third lens unit is moved from theintermediate image side to the object side and thereby the magnificationis changed from low to high.

The fourth lens unit may be constructed with a single lens. However, inorder to further reduce the total length of the objective opticalsystem, it is desirable that the entrance surface of a three-reflectionprism located in the back focus section of the objective optical systemis configured as a concave surface to thereby possess a negative powerso that the number of members is reduced. In this case, it is favorablethat an Abbe's number νp satisfies the following condition:

νp<50   (1)

When Condition (1) is satisfied, axial chromatic aberration can befavorably corrected. Also, even though Condition (1) is not satisfied,there is little problem in practical use.

If aspherical surfaces are used in at least one lens unit, that is, thethird lens unit, spherical aberration and coma can be favorablycorrected even when the lens unit is a single lens.

In order to correct diopters varying with variable magnification, it isdesirable to change the distance between the first lens unit and thesecond lens unit. In this case, either the first lens unit or the secondlens unit, or both, may be moved. It is, of course, favorable that thenumber of moving lens units is made as small as possible, because a lensmovement mechanism is simplified. Also, if the variable magnificationratio is low, the diopter will undergo little change, and thus there isno problem in practical use even when only the third lens unit is moved.

Further, it is favorable that the finder satisfies the followingcondition:

0.5<L _(pr) /L _(obj)<0.7   (2)

where L_(pr) is the optical path length of the prism and L_(obj) is themaximum optical path length of the objective optical system (a distance,measured along the optical axis, from the entrance surface of the firstlens unit to the intermediate image).

If the lower limit of Condition (2) is passed, the total length of theobjective optical system will be increased, and hence the thickness ofthe camera cannot be reduced. Beyond the upper limit, aberration isdeteriorated because the power of each lens unit is strengthened.

Subsequently, reference is made to another function of the presentinvention. The back focus section of the objective optical system isprovided with the three-reflection prism for erecting an image. Theintermediate image of the objective optical system is formed in thevicinity of the exit surface of the prism, where a field frame isplaced. Light incident on the prism from the objective optical system isreflected upward and then back by the reflecting surfaces of the prismand is further reflected laterally by the third reflecting surface. Inthis way, the intermediate image is formed in a plane nearly parallelwith the axis of incident light from the objective optical system. Inorder to further erect the image, the image needs to be once-reflectedbetween the intermediate image and the ocular optical system. Theoptical path length from the intermediate image to the ocular opticalsystem becomes nearly equivalent to the focal length of the ocularoptical system. If a prism is used as a reflecting member on the ocularoptical system side, it will have the total length for more than onereflection, and a width W of the prism becomes large, with a resultingincrease in camera width (refer to FIG. 2). Thus, the present inventionuses a one-reflection mirror.

In order to prevent the adhesion of dirt particles to optical componentssituated in the vicinity of the intermediate image, notably, to the exitsurface of the prism, an enclosed structure is required. Moreover, fordiopter adjustment, it is necessary to move the eyepiece along theoptical axis.

Thus, in the present invention, the ocular optical system is constructedwith at least two lenses, one fixed, lying on the intermediate imageside and the other moved for diopter adjustment, lying on the pupilside. Consequently, the enclosed structure can be provided, extendingfrom the exit surface of the prism, through the field frame and themirror, to the fixed lens. In this case, it is desirable that the ocularoptical system satisfies the following condition:

|f _(R2) /f _(R1)|<0.5   (3)

where f_(R1) is the focal length of one lens on the objective opticalsystem side, of two lenses constituting the ocular optical system andf_(R2) is the focal length of the other lens on the pupil side. If theupper limit of Condition (3) is passed, the variation of sphericalaberration caused by the lens movement for diopter adjustment becomesconsiderable, which is unfavorable.

The entrance surface of the prism, as mentioned above, may have anegative power to serve as a part of the objective optical system.Furthermore, the exit surface of the prism may be configured as a convexsurface to play the role of a field lens. Since the member of the fieldframe is located above the side face of the objective optical system,the height of the camera is not increased even when a mechanism memberfor changing the size of the field frame is placed.

In accordance with the drawings, the embodiments of the presentinvention will be explained below.

FIRST EMBODIMENT

In FIGS. 3A, 3B, and 3C, an objective optical system 12 in thisembodiment includes a first lens unit L₁ with negative refracting power,having concave surfaces on the object side and the pupil side; a secondlens unit L₂ of a meniscus lens with negative refracting power,directing concave surfaces toward the object side; a third lens unit L₃with positive refracting power, having convex surfaces on both sides;and a fourth lens unit L₄ with negative refracting power, having aconcave surface configured as the entrance surface of a three-reflectionprism P. An image erecting optical system 13, as shown in FIG. 4, isconstructed with the three-reflection prism P, in which a field frame Qis placed so as to come in contact with the exit surface thereof. Amirror R for reflecting incident light is located so that the axis oflight emerging from the mirror R becomes parallel with that of lightincident on the prism P. An ocular optical system 14 is constructed withan eyepiece L₅ with positive refracting power, having convex surfaces onboth sides.

When a change of the magnification of the finder is made from thewide-angle position to the telephoto position, the third lens unit L₃ issimply moved toward the object side, and the first and second lens unitsL₁ and L₂ are moved along the optical axis for diopter adjustmentinvolved in the change of the magnification. Also, each of the first,second, and third lens units is constructed with a single lens.

In the first embodiment, since the diopter adjustment is not made withrespect to an observer's eye, the ocular optical system 14 has a singlefixed lens L₅ to hermetically seal an intermediate image section.

The surfaces of individual optical components, in order from the objectside, are labeled r₁-r₁₁ in FIG. 3B. Aspherical surfaces are used for asurface r₂ on the pupil side of the first lens unit L₁, a surface r₃ onthe object side of the second lens unit L₂, both surfaces r₅ and r₆ ofthe third lens unit L₃, and a surface r₁₁ on the pupil side of theeyepiece L₅.

In the present invention, since the exit surface of the three-reflectionprism P is configured to be convex and is also used as the field lens,the number of parts can be reduced. Also, although in the presentinvention the entrance surface of the three-reflection prism P is shapedinto a concave form and is used as the fourth lens unit of the objectiveoptical system 12, it may be constructed as an independent lens withnegative refracting power.

Subsequently, numerical data of the first embodiment are shown below.Also, aberration characteristics of the optical system of the finder inthe first embodiment are as shown in FIGS. 5A-5C, 6A-6C, and 7A-7C.

In the numerical data, ω is a half angle of view of emergence (°); EP isan eyepoint; m is a finder magnification; r₁, r₂, . . . are radii ofcurvature (mm) of individual lens and prism surfaces; d₁, d₂, . . . aredistances (mm) between individual surfaces; n₁, n₂, . . . are refractiveindices of individual lenses and prisms in the d line; ν₁, ν₂, . . . areAbbe's number of individual lenses and prisms; r is a paraxial radius ofcurvature; k is a conic constant; and A₄, A₆, A₈, and A₁₀ are asphericalcoefficients of the fourth, sixth, eighth, and tenth orders,respectively. These symbols are applied to all the embodiments.

Also, the configuration of each of the aspherical surfaces is given bythe following equation:

x=(y ² /r)/[1+{1−(1+k)(y/r)²}^(½) ]+A _(4y) ⁴ +A _(6y) ⁶ +A _(8y) ⁸ +A_(10y) ¹⁰

where x is the coordinate in the direction of the optical axis and y isthe coordinate in the direction normal to the optical axis.

Magnification (m) 0.43×(wide-angle)−0.64×(middle)−1.00x (telephoto)

Half angle of view (ω) 26.0° (wide-angle)−17.1° (middle)−10.8°(telephoto)

Pupil diamater φ 4 mm

r₁=−22.013

d₁=0.800 n₁=1.58423 ν₁=30.49

r₂=12.273

d₂=6.150 (wide-angle), 3.482 (middle), 1.231 (telephoto)

r₃=−3.597

d₃=1.147 n₃=1.58423 ν₃=30.49

r₄=−9.743

d₄=1.844 (wide-angle), 0.924 (middle), 0.200 (telephoto)

r₅=7.775

d₅=2.043 n₅=1.52542 ν₅=55.78

r₆=−4.613

d₆=1.716 (wide-angle), 4.190 (middle), 8.278 (telephoto)

r₇=−17.007

d₇=23.900 n₇=1.52542 ν₇=55.78

r₈=−14.423

d₈=0.000

r₉=∞

d₉=18.000

r₁₀=28.036

d₁₀=2.600 n₁₀=1.49241 ν₁₀=57.66

r₁₁=−14.005

d₁₁=18.500

r₁₂=(EP)

Aspherical coefficients

Second surface

r=12.275, k=−1.02222

A₄=−2.89549×10⁻⁴, A₆=−5.06179×10⁻⁵,

A₈=3.39356×10⁻⁶, A₁₀=0.00000

Third surface

r=−3.597, k=−1.19309

A₄=−2.50359×10⁻³, A₆=−3.43509×10⁻⁴,

a₈=2.66297×10⁻⁵, A₁₀=−3.31788×10⁻⁶

Fifth surface

r=7.775, k=−13.22244

A₄=9.85901×10⁻⁴, A₆=−6.07753×10⁻⁵,

A₈=−1.42148×10⁻⁶, A₁₀=5.49069×10⁻⁸

Sixth surface

r=−4.613, k=−0.28412

A₄=5.47072×10⁻⁴, A₆=6.94534×10⁻⁵,

A₈=−4.73049×10⁻⁶, A₁₀=0.00000

Eleventh surface

r=−14.005, k=−3.60103

A₄=−4.86880×10⁻⁵, A₆=−2.05766×10⁻⁶,

A₈=1.08973×10⁻⁷, A₁₀=−1.73226×10⁻⁹

Values of parameters shown in Conditions (1) and (2)

Condition (1): νp=55.78

Condition (2): L_(obj)=37.6 mm, L_(pr)=23.9 mm, L_(pr)/L_(obj)=0.636

SECOND EMBODIMENT

The second embodiment is explained with reference to FIGS. 8A, 8B, and8C. This embodiment has the same arrangement as the first embodimentwith the exception that when the magnification is changed, the secondand third lens units are moved. Since the first lens unit is fixed andthe number of moving lens units is reduced, a variable magnificationmechanism can be simplified.

Subsequently, numerical data of the second embodiment are shown below.

Magnification (m) 0.43×(wide-angle)−0.64×(middle)−1.00×(telephoto)

Half angle of view (ω) 25.6° (wide-angle)−16.7° (middle)−10.4°(telephoto)

pupil diameter φ4 mm

r₁=−68.520

d₁=0.800 n₁=1.58423 ν₁=30.49

r₂=15.062

d₂=3.232 (wide-angle), 3.555 (middle), 1.069 (telephoto)

r₃=−6.164

d₃=1.041 n₃=1.58423 ν₃=30.49

r₄=−18.904

d₄=5.729 (wide-angle), 2.449 (middle), 0.800 (telephoto)

r₅=9.585

d₅=2.097 n₅=1.52542 ν₅=55.78

r₆=−6.269

d₆=0.800 (wide-angle), 3.757 (middle), 7.893 (telephoto)

r₇=−27.823

d₇=23.900 n₇=1.52542 ν₇=55.78

r₈=−17.083

d₈=0.000

r₉=∞

d₉=18.000

r₁₀=217.193

d₁₀=2.600 n₁₀=1.49241 ν₁₀=57.66

r₁₁=−10.197

d₁₁=18.500

r₁₂=(EP)

Aspherical coefficients

Second surface

r=15.062, k=−0.64061

A₄=−3.00125×10⁻⁴, A₆=−5.16751×10⁻⁵,

A₈=2.18066×10⁻⁶, A₁₀=0.00000

Third surface

r=−6.164, k=−1.21250

A₄=−6.73166×10⁻⁴, A₆=−2.42596×10⁻⁵,

A₈=−1.39647×10⁻⁵, A₁₀=9.10282×10⁻⁷

Fifth surface

r=9.585, k=−9.47791

A₄=4.12329×10⁻⁴, A₆=−5.34777×10⁻⁵,

A₈=2.91441×10⁻⁶, A₁₀=−6.22966×10⁻⁸

Sixth surface

r=−6.269, k=−0.07796

A₄=4.64493×10⁻⁴, A₆=−1.57600×10⁻⁵,

A₈=6.72748×10⁻⁷, A₁₀=0.00000

Eleventh surface

r=−10.197, k=−1.66633

A₄=−8.32536×10⁻⁵, A₆=1.55257×10⁻⁶,

A₈=−4.32080×10⁻⁸, A₁₀=3.99884×10⁻¹⁰

Values of parameters shown in Condition (1) and (2)

Condition (1): νp=55.78

Condition (2): L_(obj)=37.599 mm, L_(pr)=23.9 mm, L_(pr)/L_(obj)=0.636

THIRD EMBODIMENT

The third embodiment is explained with reference to FIGS. 9A, 9B, and9C. The objective optical system 12 in this embodiment, unlike those inthe first and second embodiments, includes the first lens unit L₁ withnegative refracting power, having a convex surface on the object sideand a concave surface on the pupil side; the second lens unit L₂ of ameniscus lens with negative refracting power, having a concave surfaceon the object side and a convex surface on the pupil side; the thirdlens unit L₃ with positive refracting power, having convex surfaces onboth sides; and the fourth lens unit L₄ having a concave surfacedirected toward the object side, configured as the entrance surface ofthe three-reflection prism P. The image erecting optical system 13 hasthe same arrangement as in the first embodiment. The ocular opticalsystem 14, unlike those of the first and second embodiments, isconstructed with a fixed lens L₆ having a concave surface on the objectside and a moving lens L₇ having convex surfaces on both sides. Thefixed lens L₆ is designed so that the intermediate image section ishermetically sealed to prevent the adhesion of dirt particles to theexit surface of the prism P. In the third embodiment, asphericalsurfaces are used for a surface r₂ on the pupil side of the first lensunit L₁, a surface r₃ on the object side of the second lens unit L₂,both surfaces r₅ and r₆ of the third lens unit L₃, and a surface r₁₃ onthe pupil side of the moving lens L_(7.)

In the third embodiment, materials that satisfy Condition (1) are usedand thus axial chromatic aberration is favorably corrected.

Subsequently, numerical data of the third embodiment are shown below.

Magnification (m) 0.43× (wide-angle)−0.64× (middle)−0.99× (telephoto)

Half angle of view (ω) 25.8° (wide-angle)−16.9° (middle)−10.8°(telephoto)

Pupil diameter φ 4 mm

r₁=9.543

d₁=0.800 n₁=1.58423 ν₁=30.49

r₂=4.794

d₂=1.578 (wide-angle), 2.622 (middle), 1.816 (telephoto)

r₃=−7.169

d₃=0.800 n₃=1.58423 ν₃=30.49

r₄=−16.728

d₄=7.512 (wide-angle), 3.193 (middle), 0.800 (telephoto)

r₅=6.681

d₅=2.710 n₅=1.49241 ν₅=57.66

r₆=−7.381

d₆=0.800 (wide-angle), 3.374 (middle), 7.272 (telephoto)

r₇=−24.314

d₇=22.900 n₇=1.58423 ν₇=30.49

r₈=−17.977

d₈=0.000

r₉=∞

d₉=15.200

r₁₀=−42.007

d₁₀=0.800 n₁₀=1.58423 ν₁₀=30.49

r₁₁=∞

d₁₁=variable (fixed in magnification change)

r₁₂=34.838

d₁₂=2.600 n₁₂=1.49241 ν₁₂=57.66

r₁₃=−9.980

d₁₃=variable (fixed in magnification change)

r₁₄=(EP)

Aspherical coefficients

Second surface

r=4.794, k=1.22930

A₄=−6.62879×10⁻⁴, A₆=−9.53119×10⁻⁵,

A₈=3.35102×10⁻⁵, A₁₀=0.00000

Third surface

r=−7.169, k=1.43673

A₄=1.68122×10⁻³, A₆=1.66187×10⁻⁴,

A₈=−1.92425×10⁻⁵, A₁₀=6.11812×10⁻⁶

Fifth surface

r=6.681, k=−0.29920

A₄=−2.41385×10⁻⁴, A₆=1.06721×10⁻⁴,

A₈=3.58275×10⁻⁶, A₁₀=−1.74078×10⁻⁷

Sixth surface

r=−7.381, k=−1.52416

A₄=8.10006×10⁻⁴, A₆=5.81412×10⁻⁵,

A₈=9.43870×10⁻⁶, A₁₀=0.00000

Thirteenth surface

r=−9.980, k=−5.25419

A₄=−5.08607×10⁻⁴, A₆=1.10983×10⁻⁵,

A₈=−2.33686×10⁻⁷, A₁₀=2.69264×10⁻⁹

Values of parameters shown in Conditions (1), (2), and (3)

Condition (1): νp=30.49

Condition (2): L_(obj)=37.1 mm, L_(pr)=22.9 mm, L_(pr)/L_(obj)=0.617

Condition (3): f_(r1)=−71.9, f_(R2) =16.0, |f _(R2)/f_(R1)|=0.223.

What is claimed is:
 1. A real image mode variable magnification finder,comprising: an objective optical system having positive refractingpower; an image erecting optical system; and an ocular optical systemhaving positive refracting power, said objective optical system having,in order from an object side, a first lens unit with negative refractingpower, a second lens unit with negative refracting power, a third lensunit with positive refracting power, and a fourth lens unit withnegative refracting power, wherein, when a magnification of said finderis changed, a plurality of lens units including said third lens unit aremoved, a distance between said second lens unit and said third lens unitis changed so as to be shorter in an intermediate position than in awide-angle position and much shorter in a telephoto position than in theintermediate position, and a distance between said first lens unit andsaid second lens unit is changed so as to be shorter in the intermediateposition than in the wide-angle position and to be much shorter in thetelephoto position than in the intermediate position.
 2. A real imagemode variable magnification finder according to claim 1, wherein saidimage erecting optical system includes a prism, and said fourth lensunit with negative refracting power is constructed to be integral withan entrance surface of said prism.
 3. A real image mode variablemagnification finder according to claim 2, wherein said prism has firstand second reflecting surfaces that are arranged such that an axis ofreflected light from said second reflecting surface is positionedsubstantially above an axis of incident light on said prism and a thirdreflecting surface constructed and arranged to orient an axis ofreflected light from said third reflecting surface in a directionsubstantially perpendicular to the axis of incident light on said prism.4. A real image mode variable magnification finder according to claim 2,wherein said prism satisfies the following condition: v _(p)<50 wherev_(p) is an Abbe's number of said prism.
 5. A real image mode variablemagnification finder according to claim 2, wherein at least one lensunit including said third lens unit has aspherical surfaces.
 6. A realimage mode variable magnification finder according to claim 2, whereinwhen the magnification is changed, a distance between said third lensunit and said fourth lens unit is changed.
 7. A real image mode variablemagnification finder according to claim 6, wherein while themagnification of said finder is changed from a wide-angle position to atelephoto position, said third lens unit is moved solely toward anobject side.
 8. A real image mode variable magnification finderaccording to claim 7, wherein said prism satisfies the followingcondition: 0.5<L _(pr) /L _(obj)<0.7 where L_(pr) is an optical pathlength of said prism and L_(obj) is a maximum optical path length ofsaid objective optical system, that is, a distance, measured along anoptical axis, from an entrance surface of said first lens unit to anintermediate image.
 9. A real image mode variable magnification finderaccording to claim 8, wherein said prism satisfies the followingcondition: 0.617<L _(pr) /L _(obj)<0.7.
 10. A real image mode variablemagnification finder according to claim 1, wherein at least one lensunit including said third lens unit has aspherical surfaces.
 11. A realimage mode variable magnification finder according to claim 1, wherein,when the magnification is changed, a distance between said third lensunit and said fourth lens unit is changed.
 12. A real image modevariable magnification finder according to claim 11, wherein, while themagnification of said finder is changed from a wide-angle position to atelephoto position, said third lens unit is moved solely toward anobject side.
 13. A real image mode variable magnification findercomprising: an objective optical system having positive refractingpower; an image erecting optical system; and an ocular optical systemhaving positive refracting power, said objective optical system having,in order from an object side, a first lens unit with negative refractingpower, a second lens unit with negative refracting power, a third lensunit with positive refracting power, and a fourth lens unit withnegative refracting power, wherein said first lens unit includes anegative meniscus lens, said second lens unit includes a negativemeniscus lens, and an object-side surface of said negative meniscus lensof said first lens unit has a larger absolute value of radius ofcurvature than an object-side surface of said negative meniscus lens ofsaid second lens unit, and wherein when a magnification of said finderis changed, a plurality of lens units including said third lens unit aremoved, a distance between said second lens unit and said third lens unitis changed, and a distance between said first lens unit and said secondlens unit is changed.